Radiation-curable adhesive compositions are employed in a variety of applications in which a durable, permanent bond or seal between parts is desired. The use of an adhesive provides a cost-effective alternative to mechanical mounts that imparts less stress on the parts being joined together. In many cases, it is possible to expose all portions of the curable adhesive composition to direct radiation, such as when the adhesive is positioned entirely within a gap between parts that are to be bonded together or sealed, or when at least one of the parts is transparent to radiation of the frequency (or wavelength) needed to cure (chemically harden) the adhesive composition. However, in many cases, it is not possible to directly expose all portions (i.e., the entire volume or mass) of the radiation-curable adhesive composition to radiation of a frequency (or wavelength) that will initiate curing of the adhesive composition (i.e., an actinic radiation), such as when at least a portion of the adhesive composition is disposed between parts comprised of a material that blocks the actinic radiation. Also, a light blocking coating may be applied over a surface of the optical element to obstruct or block radiation that would cause degradation of the adhesive. The unexposed portions of the adhesive composition may cure very slowly, resulting in unacceptably long fixing times. Accordingly, there has been a recognized need to provide structures and processes that facilitate rapid curing of those portions of a radiation-curable adhesive or sealant composition that cannot be exposed directly to the actinic radiation (e.g., along a straight line from the radiation source to the adhesive).
U.S. Patent Application Publication No. 2013/0090402 A1 describes the use of a light-diffusing optical fiber that is coupled to an actinic light source and is at least partially disposed within a portion of a radiation-curable adhesive composition that is located in a region between parts (e.g., a holder and an optical element) that are to be joined together, which region is blocked from receiving radiation directly from an external source. While providing a workable solution to the problem of exposing a radiation-curable composition that is blocked from being irradiated by an external source to actinic radiation, this technique is relatively expensive due to the added steps of providing a light-diffusing optical fiber coupled to an actinic light source, and properly positioning the light-diffusing optical fiber within the adhesive composition to facilitate exposure of the composition to actinic radiation and rapid curing.